Industrial processes for the production of mixtures of cyclohexanol and cyclohexanone from cyclohexane are currently of considerable commercial significance and are described in the patent literature. In typical industrial practice, cyclohexane is oxidized to form a reaction mixture containing cyclohexyl hydroperoxide (CHHP). The resulting CHHP is decomposed, optionally in the presence of a catalyst, to form a reaction mixture containing cyclohexanol and cyclohexanone. In the industry, such a mixture is known as a K/A (ketone/alcohol) mixture, and can be readily oxidized to produce adipic acid, which is an important reactant in processes for preparing certain condensation polymers, notably polyamides. Due to the large volumes of adipic acid consumed in these and other processes, improvements in processes for producing adipic acid and its precursors can be used to provide beneficial cost advantages.
A representative example of the oxidation of cyclohexane to CHHP can be found in Druliner et al., U.S. Pat. No. 4,326,084, in which cobalt salts are used as homogenous catalysts to form a reaction mixture containing CHHP, and for subsequently decomposing the resulting CHHP to form a mixture containing K and A.
Druliner et al., WO98/34894, Aug. 4, 1998, disclose decomposing a hydroperoxide by contacting it with a catalytic amount of a heterogeneous catalyst selected from the group consisting of Au (gold), Ag (silver), and Cu (copper). Preferably, the catalyst is supported on a solid support such as SiO.sub.2, Al.sub.2 O.sub.3, carbon, MgO or TiO.sub.2.
Komiya et al., (J. Molecular Catalysis A, 117, p 21-37, 1997) studied the oxidation of alkanes to the corresponding alcohols and ketones using molecular oxygen and copper catalysts. However, the presence of stoichiometric amounts of aldehydes is required in order to form a peracid intermediate which functions as the actual oxidizing reagent.
Pugi, K. (U.S. Pat. No. 3,530,185) teaches a process for the oxidation of cyclohexane to K and A, optionally using a soluble cobalt catalyst. However, the resulting mixture contains significant amounts of CHHP.
In view of the above, it would be desirable to have catalysts that would produce K and A directly from cyclohexane without the additional CHHP decomposition step which would result in simpler processing and less loss of product. The use of the most desirable catalysts would result in high conversion and selectivity along with little or no CHHP or high oxidation products present in the final product.
It is thus an object of the present invention to overcome some of the deficiencies of the prior art and further to provide a process for the one-step oxidation of cycloalkanes (cyclohexane) to the corresponding alcohol (cyclohexanol) and ketone (cyclohexanone) using a heterogeneous catalyst that results in little or no CHHP present in the final product mixture. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the detailed description which follows hereinafter.